He competed at the
2016 Summer Olympics as he was part of the Swedish team in the
men's team event. Mattias won two medals at the
2016 European Table Tennis Championships. Paired with
Matilda Ekholm he won a silver medal in mixed doubles, and paired with
Kristian Karlsson he won a bronze medal in men's doubles. He won the Swedish National team championship with Halmstad BTK 2018 same years as he was a part of the Swedish National team who won a bronze medal at WTTC 2018 which was played in Halmstad Arena (the same arena where Halmstad BTK won gold a month later). In April 2019, he won a silver medal in the men's singles competition during the
World Championships in
Budapest,
Hungary, after losing the final to
Ma Long from China. Karlsson's highest world rank was 7th, attained in August 2019. In 2020, Karlsson upset Xu Xin at the WTT Macau event in ITTF's restart campaign following the coronavirus pandemic.
2021 In March, Karlsson was upset by
Dimitrij Ovtcharov in the quarter-finals of the
WTT Contender event at WTT Doha. He was upset in the round of 32 by
An Jaehyun in the WTT Star Contender event. In June, Karlsson reached the semi-finals of the
2021 European Table Tennis Championships, where he was upset in the semi-finals by eventual champion
Timo Boll. Karlsson had a disappointing
2020 Olympics, losing in the round of 16 in the singles event to
Omar Assar. In the team event, Sweden defeated USA in the first round 3–1, but Falck was upset by
Kanak Jha. Sweden lost to Japan in the quarter-finals after
Koki Niwa upset Karlsson in the fourth round. Niwa later stated that Karlsson's forehand was not in good condition. Mattias and
Kristian Karlsson became World champions in the
men's doubles at the
2021 World Table Tennis Championships becoming the first Swedish duo to win gold since 1991.
2023 In August, Karlsson won WTT Contender title at Rio de Janeiro after defeating Sora Matsushima from Japan in 4-2 set score and winning the WTT title for the very first time. In September, Karlsson was playing in the Swedish National team that won the 2023 STUPA European Table Tennis Championships in Malmö. That victory marked Sweden's first team title win in 21 years. The team also consisted of
Truls Moregardh,
Kristian Karlsson and
Anton Kallberg.
Jorgen Persson was head coach. ==Playing style==